1. Field of the Invention
This invention relates to a method of recovering a gaseous product gas containing carbon monoxide and hydrogen from a subterranean, viscous oil-containing formation which has previously been exploited by in-situ combustion, and more particularly the present invention relates to injecting a combustible gas into a formation previously exploited by in-situ combustion until the formation is saturated with the gas, reinitiating in-situ combustion, injecting a mixture of an oxygen-containing gas and steam into the formation and producing a combustible product gas from the formation which may be utilized as a fuel or other purposes.
2. Background of the Invention
Increasing worldwide demand for petroleum products, combined with continuously increasing prices for petroleum and products recovered therefrom, has prompted a renewed interest in the sources of hydrocarbons which are less accessible than crude oil of the Middle East and other countries. One of the largest deposits of such sources of hydrocarbons comprises tar sands deposits found in Northern Alberta, Canada, and in the Midwest States of the United States. While the estimated deposits of hydrocarbons contained in tar sands are enormous (e.g., the estimated total of the deposits in Alberta, Canada is 250 billion barrels of synthetic crude equivalent), only a small proportion of such deposits can be recovered by currently available mining technologies (e.g., by strip mining). For example, in 1974 it was estimated that not more than about 10% of the then estimated 250 billion barrels of synthetic crude equivalent of deposits in Alberta, Canada was recoverable by the then available mining technologies. (See SYNTHETIC FUELS, March 1974, Pages 3-1 through 3-14). The remaining about 90% of the deposits must be recovered by various in-situ techniques such as electrical resistance heating, steam injection and in-situ forward and reverse combustion. In addition to tar sands, heavy, viscous crudes and crudes from partially depleted reservoirs are also recoverable by in-situ production techniques.
While details of operating of all of such in-situ techniques vary, a common objective thereof is to lower the viscosity of the hydrocarbon deposits to the point where they can be pumped to the surface of the formation with equipment normally available at the formation site.
Of the aforementioned, in-situ recovery methods, in-situ combustion (both forward and reverse) appears to be the most promising method of economically recovering large amounts of hydrocarbon deposits with currently available technology. The attractiveness of the in-situ combustion methods arises primarily from the fact that it requires relatively little energy necessary for sustaining combustion of the hydrocarbon deposits. In contradistinction, other in-situ techniques, such as electrical resistance heating and steam injection require considerable amounts of energy, e.g., to heat the steam at the surface before it is injected into the petroliferous formation.
Conventional in-situ combustion involves drilling of at least two substantially vertical wells into the formation, the wells being separated by a horizontal distance within the formation. One of the wells is designated an injection well, and the other a production well. The recovery of hydrocarbons is accomplished by raising the temperature around a bore hole to the combustion temperature of the petroliferous deposit with some type of a conventional down hole heater/burner apparatus, and then supporting the combustion by injecting an oxidizing gas, e.g., oxygen or air into the formation. There are two basic processes of in-situ combustion, viz., forward and reverse combustion. Forward combustion is initiated at the oxidant injection well and the combustion front propagates toward the production well. Reverse combustion is initiated at the production well and the combustion front propagates toward the oxidant injection well. Hydrocarbon vapors produced during the combustion process are recovered at the surface of the formation and stored in appropriate containers. The combustion is conducted at a temperature not to exceed 1500.degree. F. for about 12 months until the viscosity of oil deposits is reduced to 700-800 cp, generally considered necessary for pumping the oil to the surface of the formation. Further details of forward and reverse in-situ combustion techniques are set forth in SYNTHETIC FUELS, March 1974, pages 3-4 through 3-14, and in THE TAR SANDS OF CANADA by F. W. Camp, pages 27-34, Cameron Engineers, Inc., Denver, Col., 2nd Edition (1974), the entire contents of which are incorporated herein by reference. Modified in-situ combustion techniques using a combination of oxygen and other chemical substances are also known in the art. For example, Heilman et al., U.S. Pat. No. 2,718,263 uses a mixture of oxygen-containing gas and fuel to generate heat in the formation, and Elzinga, U.S. Pat. No. 3,087,541, injects fuel into the formation only after the combustion has started. Both of these modified in-situ prior art combustion processes use fuels injected externally into the formation either simultaneously with oxygen or after the injection of oxygen to control the direction of speed of propagation of the combustion front.
After the maximum amount of hydrocarbon has been recovered by an in-situ combustion operation, there remains in the formation a considerable amount of hydrocarbons, particularly solid hydrocarbon materials in the form of a coke like residue distributed on the formation matrix. A method for converting such solid hydrocarbons to a combustible gas consisting predominantly of gaseous carbon monoxide and hydrogen within the formation by injecting an oxygen-containing gas and steam into the formation and recovering the combustible gas therefrom which may be utilized as fuel or feed gas for manufacturing operations is described in U.S. Pat. No. 4,026,357 to Redford.
U.S. Pat. No. 4,397,352 to Audeh discloses an improved in-situ combustion process for the recovery of oil from tar sand formations wherein a combustible gas is introduced into the formation prior to in-situ combustion.
Accordingly, it is a primary object of this invention to provide an improvement in the prior art known process for gasification of a subterranean, viscous oil containing formation previously exploited by in-situ combustion so as to produce a combustible gas in the formation consisting predominantly of carbon monoxide and hydrogen that is recovered.